Radiographic images such as X-ray images are employed in many fields, for example, for use in diagnosis of an illness. There have been employed, as a method for obtaining X-ray images, so-called radiation photography systems in which X-rays which have passed through a photographic subject are irradiated onto a phosphor layer (fluorescent screen) to form a visible light, which is irradiated onto a silver halide photographic material (hereinafter, also denoted simply as a photographic material) similarly to conventional photography, followed by being subjected to photographic processing to obtain a visible silver image.
Recently, there was developed a new method for obtaining an image directly from a phosphor layer in place of an image forming method by use of silver halide photographic materials. In such a method, radiation having passed through a subject is absorbed by a phosphor and then, the phosphor is excited by light or heat energy, whereby a radiation energy accumulated in the phosphor through absorption is radiated as fluorescence and this fluorescence is detected to form an image.
Specifically, there is known a radiation image conversion method by use of photostimulable phosphors, as described in, for example, U.S. Pat. No. 3,859,527 and JP-A No. 55-12144 (hereinafter, the term JP-A refers to Japanese Patent Application Publication). In this method is used a radiation image conversion panel having a phosphor layer containing a photostimulable phosphor. Radiation having passed through a subject is irradiated onto the photostimulable phosphor layer, in which radiation energy corresponding to radiation transmission densities of the individual portions of the subject is accumulated, thereafter, the photostimulable phosphor is excited in time series by an electromagnetic wave (exciting light) such as a visible ray or an infrared ray, whereby the radiation energy accumulated in the photostimulable phosphor is emitted in the form of a stimulated emission. Signals due to the thus emitted light are, for example, photoelectrically converted to obtain electric signals. The thus obtained electric signals are reproduced as a visible image on a recording material such as a photosensitive material or on a display such as a CRT.
The foregoing radiation image reproduction posesses an the advantage that a radiation image with extensive information is obtained by much less radiation exposure, compared to conventional radiography of the combination of a conventional radiographic film and an intensifying screen.
Such a radiation image conversion panel using a photostimulable phosphor accumulates radiation image data and emits accumulated energy by scanning with an exciting light, so that accumulation of radiation images is again feasible after scanning, enabling repeated use. Thus, conventional radiography consumes radiographic films for every photographing but on the contrary, the radiation image conversion method, which repeatedly uses a radiation image conversion panel, is advantageous in terms of resource protection and economical efficiency.
Further, recent diagnostic image analysis requires a radiation image conversion panel of further enhanced sharpness. There were attempted means for improving sharpness, for example, controlling the shape of the formed photostimulable phosphor particles to achieve improvement in sensitivity and sharpness.
Such attempts included, for example, a method of using a photostimulable phosphor layer formed of fine pseudo-columnar blocks deposited on a support having a finely concaved pattern, as described in JP-A No. 61-142497.
There were also proposed the use of a radiation image conversion panel having a photostimulable phosphor layer in which cracks between columnar blocks obtained by depositing a photostimulable phosphor on a support having a micro-pattern were further developed by being subjected to a shock treatment, as described in JP-A No. 61-142500; the use of a radiation image conversion panel in which cracking is caused on the surface of a photostimulable phosphor layer formed on a support to form pseudo-columns, as described, for example, in JP-A No. 62-39737; and a method in which a photostimulable phosphor layer having voids is formed on the support surface through vapor deposition and then subjected to a heat treatment to grow the voids to form cracks, as described in, for example, JP-A No. 62-110200.
There was also proposed a radiation image conversion panel having, on a support, a photostimulable phosphor layer formed of finely long columnar crystals exhibiting a given inclination to the vertical line of the support, as described in, for example, JP-A No. 2-58000.
Recently, there was proposed a radiation image conversion panel using a Eu-activated photostimulable phosphor comprised of an alkali halide such as CsBr as a parent material. Specifically, the use of Eu as an activator enabled to render it feasible to achieve enhanced X-ray conversion efficiency which was never realized before.
There is also known a technique intending to enhance sensitivity in which at least one vaporization source containing a phosphor parent component and an activator component of EuOX or a mixture of EuOX and EuXm was vaporized under an atmosphere having a prescribed oxygen partial pressure, as described in, for example, JP-A No. 2004-233134.
JP-A No. 2004-170405 disclosed a prescribed relational expression of a molar ratio of europium/cesium and a substrate temperature during deposition as a condition for preparation of a radiation image conversion panel of high sensitivity but was silent with respect to temperature variation rate of the substrate.